The structure of the Carajلs N-4 ironstone deposit and associated rocks: relationship to Archaean strike-slip tectonics and basement reactivation in the Amazon region, Brazil

The N-4 banded ironstones of the Serra dos Carajás region, Brazil, form the largest iron-ore deposit currently being mined in South America. The ironstones belong to the Grão Pará Group, part of a discontinuous cover succession of Late Archaean to Phanerozoic volcanic and sedimentary rocks of low metamorphic grade that rest unconformably on older high grade metamorphic basement. In the region of the N-4 plateau, the rocks are disposed in a NW-trending deformed zone within a regional E-W-trending belt of low temperature ductile and brittle deformation, the Carajás fault system. Deformation of the ironstones and associated volcanics occurred predominantly during a single phase of folding reflecting NE-SW compression. This deformation has controlled the present form of the N-4 ironstone body which now lies in a major NE-verging, shallowly WNW-plunging fold pair with steeply SW-dipping axial surfaces. The compositional banding in the ironstones preserves primary sedimentation structures and there is no evidence for the pervasive mylonitisation reported previously. Exposed ironstone — volcanic rock contacts are deformed and display evidence for strain partitioning during sinistral transpression. This is thought to reflect differences in the mechanical responses of the two main rock types during deformation. A model is proposed in which the deformation of the N-4 plateau ironstones and associated rocks occurred due to transpression in a restraining bend in the Carajás Fault during regional sinistral movements. In detail, the folding affects an area that lies adjacent to, but separate from a major bend in the Carajás Fault. This suggests that deformation may have occurred due to buttressing against a pre-existing fault branch of the main Carajás Fault. This work illustrates that the distribution and patterns of deformation of low grade cover sequences in the Amazon region, including ironstones, are controlled significantly by movements along Precambrian strike-slip faults that reactivate earlier basement fabrics.